Deprecate RACSequence and RACStream

_Depends on #910 (to avoid project file conflicts)._

Resolves #890 and #913.

**To do:**

 - [x] Update standalone documentation
 - [x] Rewrite `rac_sequence` tests to focus on `rac_signal` extensions instead
 - [x] [CHANGELOG](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/deprecate-sequences/CHANGELOG.md#sequences)